As technical fields in industry become highly specialized, special high-level performance is now required of the materials used in each technical field. For example, cold-rolled steel sheet which is used after being formed into various shapes by press forming or the like is often required to have a high strength. Therefore, the use of high-strength cold-rolled steel sheet is now being envisaged. In particular, increasing the fuel efficiency of automobiles by decreasing the weight of vehicle bodies in order to protect the global environment is becoming an important topic. Accordingly, there is an increasing demand for high-strength cold-rolled steel sheet which makes it possible to achieve a decrease in the thickness of steel sheet for automobiles.
Steel sheet used for outer panels of automobiles such as outer panels of doors and for fenders requires dent resistance, i.e., the property that permanent deformation does not take place if the sheet is pressed with a finger or hit by a pebble. Dent resistance increases as the yield stress after paint-baking subsequent to press forming increases and as the sheet thickness increases. Therefore, if steel sheet having a high yield stress is used as an outer panel of an automobile, the desired dent resistance can be achieved even if the sheet thickness is decreased.
Steel sheet used for outer panels of automobiles is also required to have the ability to conform to a press die during press forming and to develop little springback when a formed article is released from a press die, i.e., to have good shape fixability. Therefore, steel sheet used in outer panels of automobiles is required to have a low yield stress before press forming.
Thus, a steel sheet for an outer panel of an automobile is required to have a low yield stress before press forming and a high yield stress after being subjected to press forming and paint-baking (painting and baking).
Bake hardenable steel sheet (BH steel sheet) is known to have such properties. BH steel sheet is steel sheet which utilizes the so-called strain age hardening phenomenon in which the yield stress increases due to solid solution C and N atoms segragating to dislocations and locking the dislocations. If BH steel sheet is used as steel sheet for an automobile, dislocations which are introduced at the time of press forming are locked by solid solution C and N at the time of paint-baking, so the yield stress after paint-baking increases. Improving the bake hardenability of high-strength steel sheet also leads to an improvement in dent resistance and shape fixability.
There have been many proposals concerning BH steel sheet. For example, Patent Documents 1 and 2 disclose a manufacturing method for a BH steel sheet having excellent deep drawability by adding Ti and Nb to ultra-low carbon steel and further adding Si, Mn, and P to increase tensile strength. However, this method has the following problems (a)-(c).
(a) In order to increase tensile strength, solid solution strengthening elements such as Si, Mn, and P are added, resulting not only in an increase in tensile strength but also M an increase in yield stress. As a result, shape fixability deteriorates, and surface distortion more easily develop.
(b) It is difficult to achieve both bake hardenability and anti room temperature aging property. In order to guarantee anti room temperature aging property, the amount of bake hardenability is restricted.
(c) At the time of press forming, linear surface defects easily develop. Namely, surface defects which develop when carrying out press forming of a BH steel sheet often are linear surface defects in the form of recesses and indentations, and they do not disappear after painting. Such surface defects become a serious problem if they develop in an outer panel of an automobile requiring a beautiful appearance such as a roof or hood or door.
Patent Documents 3-5 disclose a method of manufacturing a low carbon Al-killed steel sheet having a dual phase structure in which martensite is dispersed in ferrite (referred to below as a dual phase steel sheet). This dual phase steel sheet has a high tensile strength and a low yield stress. In addition, it does not undergo room temperature aging even when bake hardenability is large and has excellent ductility. Above-described problems (a) and (b) can be solved by using this dual phase steel sheet, but problem (c) cannot be solved.
In order to prevent this type of surface defect, Patent Documents 6 and 7 disclose an invention which decreases surface defects in a P-added cold-rolled steel sheet, Patent Document 8 discloses a method of manufacturing a Ti, Nb-added ultra-low carbon steel sheet which has excellent surface qualities, and Patent Document 9 discloses a method of manufacturing a very-low carbon cold-rolled steel sheet having excellent surface qualities.
Patent Document 1: JP S59-31827 A1
Patent Document 2: JP S59-38337 A1
Patent Document 3: JP S55-50455 A1
Patent Document 4: JP S56-90926 A1
Patent Document 5: JP S56-146826 A1
Patent Document 6: JP H11-6028 A1
Patent Document 7: JP H11-335781 A1
Patent Document 8: JP H09-227955 A1
Patent Document 9: JP H09-125161 A1